(a) Field of the Invention
The invention regards broadcasting of multimedia presentations, or signals (such as MPEG-4 presentations), in a broadcasting system providing for random access to a presentation (e.g. MPEG-2).
To allow such a random access, a given presentation has to be broadcast several times, so that several terminals can get the beginning of a presentation at different instants.
Broadcasting MPEG-4 presentations over MPEG-2 transport channels has recently been described in the Amendment 7 to MPEG-2 system.
However, it appears that this Amendment 7 is not sufficient, and requires being updated. The invention try and build MPEG-4 presentations then to see whether we could achieve or not sending them according to Amendment 7. This has lead us to point out here and there some functionality that was left aside or impossible to be efficiently handled.
We tried then to modify as minimally as possible Amendment 7 so as to be able to achieve a complete transmission of an MPEG-4 presentation. This process has been incremental and this document tries to sum up both what can't currently be done in terms of unavailable functionality and our conclusions so as to encompass complete MPEG-4 functionality in the broadcast of MPEG-4 presentations over MPEG-2 transport channels. (In the following lines, MPEG-2 will mean MPEG-2 transport.)
(b) Description of Related Art
1 Prior art: Audit of Amendment 7.
1.1 Implications on MPEG-4 Elementary Streams.
One feature of MPEG-2 is that it enables random access to a presentation. Let us focus on the implications of this when transmitting an MPEG-4 presentation according to MPEG-2.
Most current MPEG terms are defined in Annex 1.
An example of MPEG-4 presentation is given in Annex 2.
Annex 3 relates to MPEG-4 over MPEG-2 mapping according to prior art.
1.1.1 BIFS ES.
Assumption: Random access to a presentation provides two different users with the same presentation.
Assertion: Access units of a BIFS Command ES that are not RAP should not modify the scenegraph structure.
Reason: Otherwise two different users don't have the same presentation at the same point in time. More precisely: since there is random access to the presentation, we can assume there is a RAP AU after such structural modification. The RAP is necessarily a SceneReplace, and the SceneReplace is the modified scenegraph. Consequently the two users don't experience the same presentation.
Conclusion:
a) Updates are only used for scene structure construction and not for presentation purposes. Thus presentations can't be very dynamic. Many MPEG-4 presentations can't be transmitted over MPEG-2. Cartoons, for instance, would not be properly handled.
b) Even in the case of a static presentation, this implies that resources are available or will be available in a very short time which is not possible due to bandwidth limitations if the presentation is a large one. This will result in poor quality presentations (missing images that will pop up some time later, . . . )
1.1.2 OD ES.
Assumption: Random access to a presentation provides two different users with the same presentation.
Assertion: Access units of OD ES that are not RAP should not modify the stream structure.
Reason: Otherwise two different users don't have the same presentation at the same point in time. (see BIFS)
Conclusion: Since OD do not imply modifications in the presentation, this does not alter the presentation.
1.1.3 JPEG, MPEG-J
Assertion: JPEG and MPEG-J have to be sent in PES.
Reason: see Amendment 7 only BIFS & OD can be carried in sections.
Conclusion:                a) Existing error detection mechanism in sections can't be used. It can be critical to know if a JPEG or an MPEG-J AU or corrupted or not.        b) Behaviour of an MPEG-4 terminal that receives duplicate Access Units for JPEG or MPEG-J is not specified.(or not allowed)        
1.1.4 Audio & Video
Assertion: Audio and Video have to be sent in PES.
Reason: see Amendment 7 only BIFS & OD can be carried in sections.
Conclusion:                a) Audio “jingles” and “beeps” as well as “video clips” can't be properly handled        b) Behaviour of an MPEG-4 terminal that receives duplicate Access Units for Audio, Video is not specified. (or not allowed)        
1.2 Some Remarks.
In normal use-cases, we want users to experience the same visual animations in part of the presentation and not for others. Amendment 7 limits the scope of MPEG-4 presentations to an almost static BIFS scene and streamed audio and video.
Applications such as cartoons, lightweight ads could not be properly handled.
1.3 Section Versioning.
MPEG-2 allows to give a version number to sections. However the semantics of version_number is signalling a state change if it is incremented.
With the RAP mechanism mentioned above, this does not allow using the update mechanism provided by MPEG-4. As a matter of fact, a session that begins will take the first available section and then incrementally all next version-numbers. This allows a very limited and complicated way of dealing with the update mechanism.
1.4 US-A-5 477 263
This document discloses a video distribution technique, which can be implemented in TIPEG-2 and which also provides fast forward, fast reverse and channel pause functionalities to the user. Such functionalities are achieved by memorizing several versions having staggered starting times of a given program. A pointer to the program which should be decoded for presentation to a given user is also memorized. When a user indicates that he requires a special function, the pointer is changed to point to a program with a different starting time.
A drawback of this technique is that it is resource-consuming, since it requires that several versions of several different programs be stored and handled at the same time.